In recent years, the reverse osmosis process, that is, a process wherein aqueous solutions containing solutes are brought into contact with semipermeable membranes under pressure maintained above the osmotic pressure exhibited by said aqueous solutions, thereby to selectively separate the solutes contained in said aqueous solutions, has become attractive for utilization in desalination of sea water or saline water, for the recovery of useful or valuable materials from waste liquids, etc. It is possible with such a process, which does not involve any phase transition nor chemical change, to realize great savings in energy and resources as compared with, for example, the evaporation method which has hitherto been a typical technique of desalinating sea water. Moreover, with this technique of separating solutes from a liquid by reverse osmosis, it is possible to separate organic compounds having markedly lower molecular weights than with the ultrafiltration method, or the like. Additionally because the operation of such a separation can be carried out under the so-called "closed" system, the process is considered to be advantageous from the viewpoints of the prevention of environmental pollution and of the betterment of the working environment.
A number of studies have been carried out in attempts to develop semipermeable membranes which satisfy practical requirements in respect of their selective separation performance; but, heretofore, semipermeable membranes that have been put to practical use may be said to be confined to those fabricated from either one of these two kinds of polymers: cellulose acetate or cellulose esters and polyamides, that is, the "Loeb" type membrane made of cellulose acetate by processes such as described in U.S. Pat. Nos. 3,133,132 and 3,133,137 or the semipermeable composite membranes which are prepared from polyamides by processes such as described in U.S. Pat. No. 3,951,815.
However, the cellulose acetate "Loeb" type membranes are restricted in use and in processability, largely because the membranes must be kept wet at all times, that is, their capability as reverse osmosis membranes is lost once the membranes are dried. These membranes have also exhibited deficiencies such as alkali or acidic degradations and biological degradation resulting in a short life. Also, these membranes are not used widely in the separation or recovery of useful materials from liquid mixtures containing organic chemicals, because the membranes have a low selectivity for useful materials. On the other hand, the prior art composite membranes have been generally subject to deficiencies such as compaction, resulting in short life, as well as undesirably low solute rejection or flux, all resulting in an inefficient operation.
Moreover, although hitherto known, semipermeable membranes including the aforesaid cellulose acetate and polyamide membranes, exhibit a fairly good selective separation capability for desalination of sea water or brackish water, their capabilities as a reverse osmosis membrane for the selective separation of aqueous solutions containing such useful materials as various kinds of organic compounds, transition metal compounds, etc., is very limited and serves for practical purposes merely with a few kinds of useful materials. They cannot be said, as a matter of fact, to meet practical requirements in respect to their performances with several, especially more than ten, kinds of useful materials.
Unlike the case of desalinating brackish water or sea water, semipermeable membranes which are intended for use in the reverse osmosis separation of aqueous solutions containing useful materials are required to have the undermentioned performance characteristics; and if these requirements are not met, it is difficult to uniformly separate quite a variety of useful materials from the aqueous solutions thereof.
(1) The membrane should exhibit a high level of selective separation capability as well as a high flux rate with various kinds of useful materials. PA1 (2) Since the osmotic pressure of an aqueous solution containing useful materials increases in proportion to the higher degrees of concentration of useful materials, it is necessary to carry out the operation under high pressures. Accordingly, the semipermeable membrane should be capable of standing such an operation under high pressures. PA1 (3) It should have an excellent resistance to heat, and should be free from swelling, chemical metamorphism, etc., by the solution being treated. PA1 (4) It should exhibit a selective separation performance at a certain fixed level over a wide range of pH used. PA1 wherein A is selected from an alkylene radical containing from 2 to 5 carbon atoms; PA1 B is selected from an alkylene radical containing from 1 to 17 carbon atoms; PA1 X is ether (--O--) and/or ester ##STR4## linkages; n is zero or integer, when n is more than 1, B may be the same or different; PA1 R' is selected from the group consisting of hydrogen, and an alkyl radical containing from 2 to 5 carbon atoms with functional hydroxyl group and/or functional glycidyl group; PA1 and the residual one of R.sub.1, R.sub.2 and R.sub.3 is selected from the group consisting of hydrogen, an alkyl radical containing from 2 to 5 carbon atoms and the above R group represented by the Formula (II). PA1 (1) It can be effectively utilized, in the chemical industry for the separation and recovery of various kinds of organic compounds contained in aqueous solutions that arise in the process of manufacturing such organic compounds, as well as in various kinds of manufacturing processes using organic chemicals, removal of unwanted ingredients, that is, impurities, from aqueous solutions containing such organic compounds for the purpose of refining. For instance, in the synthetic fiber manufacturing processes where large amount of fiber-producing materials and solvents, such as .epsilon.-caprolactam, ethylene glycol, terephthalic acid, dimethylsulfoxide, dimethylformamide, etc., are used; and, in waste liquids that are discharged in the processes contain said fiber-producing materials and solvents at irregular concentrations. In the treatment of these waste liquids, it is possible to apply the reverse osmosis process only when a semipermeable membrane possesses chemical resistance. The present invention not only makes such possible, but also permits the recovery and reutilization of said fiber-producing materials and solvents. Also, where waste water treatments are carried out under the so-called "closed system", the method of the invention is very advantageous from the viewpoint of the prevention of environmental pollution. PA1 (2) The utilization of lower alcohols, such as methanol or ethanol, formed by fermentation and decomposition of natural products, for example, the utilization of biomass is attracting a good deal of public attention as an energy source in the future taking the place of the oil resources. Since these lower alcohols from biomass are obtainable only as their aqueous solutions at a low concentration of a mere several percent, it is impossible to utilize them in that form are as a souce of energy, like fuel, and it is necessary to resort to some means of concentrating them. This invention makes it possible, by virtue of its characteristics of selective separation in an energy-saving manner, to concentrate these aqueous solutions of lower alcohols at low concentrations very advantageously by a simple and easy process. PA1 (3) With the metal plating industry, the treatment of waste plating solutions and of liquids used for the cleansing of the products is a big problem. For such treatment to recover expensive transition metals as gold, silver, copper, nickel, chromium, cobalt, etc., and to eliminate such poisonous substrates as boric acid, prussic acid, etc., complex processes and a great deal of energy are required. The method of this invention makes it possible to simplify, by a large measure, such processes of treating waste plating solutions and waste liquids arising from cleansing of the products, and to also realize a great saving in energy. Moreover, its adoption brings about a great advantage in the aspect of the prevention of environmental pollution and the improvement of working environment, because the processes can be carried out under the so-called "closed system". Furthermore, while a large amount of water is used for the treatment of waste plating solutions or cleansing liquids, as above, the application of the method of this invention permits the recovery and reutilization of water and thus realizes a great saving in water used. This is another advantage of the method of this invention. PA1 (4) Still further, it is common, in conventional waste water treatment procedures, that the kind of low molecular weight organic compounds and transition metal compounds are not limited to just one, but several kinds of compounds are contained in the waste water being treated. Since the semipermeable composite membrane used in this invention exhibits its capability of selective separation with at least ten kinds of useful materials, it is possible to apply the method of the invention also to such waste water treatment, for instance, treatment of brackish water or sea water containing useful materials, as was previously indicated.
Furthermore, with the membrane used in the art of selective separation of aqueous solutions containing useful materials, it is thought that, in addition to the characteristics required as indicated above, its physical structure, as well as its chemical properties, is also closely related to its performance. In reality, however, the selective separation mechanism of such a membrane has not yet been theoretically clarified in definite terms. As such, it is necessary to conduct measurements and evaluations of the selective separation characteristics of semipermeable membranes for the selective separation of useful materials with respect to each individual useful material, while, needless to say, such attempts are also to be made in connection with semipermeable membranes for desalination of brackish water or sea water.
The known semipermeable membranes for reverse osmosis which have been evaluated as to their performance in selective separation of useful materials as indicated above include, besides the aforesaid cellulose acetate and polyamide semipermeable membranes, those fabricated polyamide/urea membranes, so-called "NS-100", of furfuryl alcohol, generally called "NS-200", of sulfonated polyphenyleneoxide and of polybenzimidazole. Their selective separation performances have been evaluated with respect to about thirteen kinds of low molecular weight compounds including acids, aldehydes, amines, amides, esters, ethers, ketones, phenols etc. However, the overall solute separation performance (rejection), with respect to these thirteen kinds of useful materials, of such membranes are very low, being no more than 12 to 26% with cellulose acetate membrane, 50 to 60% with polyamide membrane, and 70 to 78% with "NS-100" and "NS-200" membranes (E. Chian et al., "Environmental Science and Technology", vol. 10, No. 4, p. 364 (1976); J. E. Cadotte et al., "Office of Saline Water Research and Development Progress Report", No. 982 (1974) and "Office of Water Research and Technology", OWRT/S-1976/2 (1975)).
There is a further report on a study from a more theoretical standpoint of the selective separation performance of cellulose acetate and polyamide membranes with respect to organic compounds similar to the aforesaid useful materials; and it is reported that the results obtained indicate they do not exhibit a selective separation performance that serves practical purpose with respect to low molecular weight organic compounds (S. Sourirajan et al., "Journal of Applied Polymer Science", vol. 19, p. 801 (1975)).